My starting point with pasta, portionwise, is 100g dried weight of pasta per person for a main, on average; there are variables, of course, appetite and age chief among them. Other factors that come into play, when it comes to weight out pasta, are what – if anything – else is being eaten and which kind of sauce partners it.
But before we go any further, I must tell you about the Italian technique of holding back some pasta-cooking water to help bind a sauce to the pasta. Please, promise me that you will get into the habit of doing this every time you cook pasta. Indeed, you should make yourself incapable of draining pasta without first lowering a small cup into the cooking water to remove and reserve some for the sauce.
If it makes your life easier (not too much bubbling away on the stove), when you’re feeding a lot of people, you can follow a pasta-cooking tip from Anna Del Conte: the Vincenzo Agnesi method, which reduces the risk of overcooking and is as follows. Bring your water to the boil, add salt, then tip in the pasta, stirring well to make sure it’s all in and not clumped together. Once the water comes back to the boil, let the pasta cook for 2 minutes, then turn off the heat, cover the pan with a clean, thin tea towel (not a waffle-textured one) and clamp on a tight-fitting lid. Let the pasta stand like this for as long as the packet tells you to cook it normally. When the time is up, drain the pasta, remembering to remove a small cupful of cooking water before doing so.
My only remaining word of wisdom on the subject is also from Anna Del Conte and it is that the water you cook pasta in should be as salty as the Mediterranean. Contemporary dietary mores could not run more counter to such a recommendation; you, of course, are free to act on my advice or ignore it as you see fit.